Conventional liquid developers for electrostatic photography are produced by forming a dispersion of a resin and a dye or pigment in a non-polar solvent, e.g., an aliphatic hydrocarbon and agitating it with a ball mill or high-speed stirrer. The liquid developer thus-produced has low dispersion stability and storage stability. Therefore, with the passage of time the toner particles are precipitated to reduce the characteristics of the developer. An attempt has been made to add a large amount of dispersion stabilizer to improve the stability of the dispersion. However, this process decreases the resistance of the liquid developer and thus fails to produce a liquid developer having good properties.
U.S. Pat. No. 3,990,980 describes a developer that is designated a second liquid toner and which is prepared by the "polymerization granulation process". According to this process, a substance which dissolves in a non-polar solvent when it is a monomer and which does not dissolve in the solvent when it becomes a polymer is dissolved in the non-polar solvent together with a polymerization initiator, and the monomer in solution is polymerized to form a liquid developer having the resin particles dispersed in the solvent. The developer obtained by this method is known to have good dispersion stability and storage stability.
A method for producing colored toner particles using this polymerization granulation process is described in Japanese Patent Application (OPI) No. 52588/78 (the symbol (OPI) as used herein means an unexamined published Japanese patent application). According to this method, there is prepared a solution which contains a monomer which is soluble in a non-polar solvent when it is a monomer and which becomes insoluble when it is a polymer, a dye which is soluble in the monomer but insoluble in the non-polar solvent, and a polymerization initiator. Polymerization is effected by adding the solution dropwise into a non-polar solvent containing, say, a charge regulator. In this method, as soon as the solution is added dropwise to the non-polar solvent, the monomer starts to dissolve in the solvent to thereby form fine crystals of the dye before the monomer is polymerized. The resulting liquid developer for electrostatic photography is such that the polymer particles are adsorbed on the fine dye crystals.
However, our experimental studies have revealed that the process of Japanese Patent Application (OPI) No. 52588/78 has several problems. For one thing, many dyes have such a chemical structure that a radical can remain stable, for example, phenolic hydroxyl group, phenylamino group or nitrophenyl group, and as suggested by the fact that many of the dyes having such a structure are used as a radical-polymerization inhibitor, they operate as a substance which retards the radical polymerization of monomers. Therefore, as in the process described above, if the radical polymerization of the monomer is performed in the presence of a dye, no satisfactory polymerization is achieved, and in an extreme case, no polymer particles are deposited on the surface of the fine dye crystals. Hence, the dye that can be used in this process should not work as an inhibitor of radical polymerization. Furthermore, as indicated above, the dye must be insoluble in the non-polar solvent and soluble in the monomer to be polymerized. This means the latitude in selection of applicable dyes and monomers is very small, and in practice, many dyes cannot be used in this process.